The present invention relates generally to diagnostic imaging and, more particularly, to correcting motion errors in imaging data acquired from an object prone to motion.
Various imaging modalities are useful to image objects in or prone to motion, such as the heart in cardiac studies. For example, in computed tomography (CT), magnetic resonance imaging (MRI) and other imaging modalities directed to the acquisition of data from an object prone to motion, one or more motion correction techniques are generally used to reduce motion-induced artifacts in the reconstructed images. In known studies, this motion correction or compensation can add significant complexity in post processing of the images.
In one specific example, CT imaging requires measurement of more than 180 degrees of projection data to formulate an image. Because of various limitations in conventional CT scanners, the time necessary to collect a complete set of projection data is significant relative to object motion. For example, cardiac CT imaging is typically performed with the aid of an electrocardiogram (ECG) signal, which is used to synchronize data acquisition and image reconstruction with the phase of minimal cardiac motion. The ECG signal collected from the patient represents the electrical properties of the heart and is helpful in identifying the quiescent period of cardiac activity, which is preferred for data acquisition. Moreover, the ECG signal assists in identifying this quiescent period over several cardiac cycles. By synchronizing data collection with the quiescent period of the cardiac cycle, image artifacts and spatial resolution due to heart motion are reduced. Additionally, by consistently identifying this quiescent period in successive cardiac cycles, inconsistencies between images acquired at different cardiac cycles are reduced. ECG signals can be used similarly in MR and other imaging modalities. The ECG signal can gate acquisition of projection (known as prospective gating) or may be used subsequent to data acquisition (known as retrospective gating) to identify the phase of the cardiac cycle with minimum motion. Prospective gating allows dramatic reduction in dose administered to the patient since projection data is not collected during phases of the heart having significant organ motion.
The conventional ECG gating described above does not provide mechanical motion detection. That is, while an ECG signal can indicate that motion is occurring, has occurred, or is about to occur, it is a boundary measurement (electrical signals within the heart are measured on the surface of a patient) and does not provide accurate real-time displacement data of the heart. Instead, mechanical motion of the heart must be inferred from the electrical activity measured in the ECG signal. Since actual mechanical motion, or displacement, of the heart contributes to sub-optimal image quality, cardiac images that depend solely on ECG signals often require significant post processing to correct for motion artifacts or often require a very high acquisition rate to minimize the extent of cardiac motion during acquisition.
CT reconstruction typically does not utilize a priori information on heart motion. In conventional ECG-gated cardiac CT studies, the heart is presumed to be a stationary object during the short acquisition period identified as the quiescent period in the acquired ECG signal (applicable to both prospective and retrospective gating techniques). Conventionally, half-scan imaging techniques (requiring a fraction of the time for one complete rotation of the gantry about the subject) are used to reduce the impact of motion; however, their effectiveness is less than optimal since half-scan weighting techniques used for image reconstruction combine CT projection data acquired at both extents the of angular range of data acquisition covering 180 degree plus the fan angle of the X-ray beam. The interpolation of projection data at both ends of the dataset is predetermined and, therefore, does not change based on each particular acquisition as needed since there is no a priori information available. For data collected roughly in the center of the angular range of projection data acquisition, the data is incorporated without any weighting. Further, even with a gantry speed of 0.3 s/rotation, the central region of the projection range constitutes a slightly larger than 150 ms temporal window, which is prohibitively slow to sufficiently “freeze” cardiac motion. The data acquisition window for CT systems having dual tube-detector assemblies is typically between 70 ms and 80 ms during which heart motion may occur. Ideally, a temporal window of 20 ms to 40 ms is needed to adequately freeze cardiac motion.
It would therefore be desirable to synchronize data acquisition and image reconstruction with cardiac phase and motion data to acquire and reconstruct substantially motion-free datasets and images.